1. Field of the Invention
The present invention relates to an instrument for measuring a concentration of carbon dioxide contained in a respiratory gas by non-dispersive infrared method.
2. Related art
This type of instrument is called a capnometer. A typical example of the known capnometer is the non-dispersive infrared radiation analyzer. In the capnometer, to measure a concentration of CO.sub.2 gas contained in the respiratory gas, infrared radiation is emitted from infrared radiation source, and passed through the respiratory gas. The concentration of CO.sub.2 gas can be measured by passing a beam of infrared radiation through the gas, and ascertaining the attenuation of the intensity of infrared radiation in a narrow wavelength band which is remarkably absorbed by CO.sub.2 gas. A wavelength of approximately 4.3 .mu.m is used for this purpose as a measuring wavelength, and a wavelength of approximately 3.7 .mu.m which is not absorbed by the carbon dioxide is used as a reference wavelength. As known, a relation between the concentration of CO.sub.2 gas and an intensity of light is shown by the Lambert-Beer relation, and is given by EQU Iout=Iin exp (-kcl)
where
Iin: intensity of light going into the sample. PA1 Iout: intensity of light coming out of the sample. PA1 k, c, l: absorption coefficient, concentration of CO.sub.2 gas, and optical length respectively.
The equation shows that a concentration c of CO.sub.2 gas can be measured if the Iin, Iout, k and l are known.
The capnometer based on above principle is disclosed in U.S. Pat. No. 5,153,436. A schematic illustration of the analyzer is shown in FIG. 4. In the figure, reference numeral 30 is a housing of a measuring section, and 31 is an airway adaptor used for introducing respiratory gases of a patient into the analyzer. The airway adaptor 31 is inserted directly in the flow path between the ventilator and the endotracheal tube (not shown), which is extended in the directions vertical to the paper surface of the drawing. Windows 32 and 33 are respectively formed in both sides of the airway adaptor 31. These windows are made of sapphire having a good transparency to the infrared radiation. The airway adaptor 31 is firmly held in a receptacle portion 34 of the housing 30 in a detachable fashion. The airway adaptor 31 may be the reusable type or the disposal type.
An infrared radiation source 35 is disposed in the left hand of the receptacle portion 34. A light beam is emitted from the infrared radiation source 35, passes through a sapphire window 34a disposed in proximity to the left hand of the receptacle portion 34, and the windows 32 and 33 of the airway adaptor 31 and a sapphire window 34b disposed in proximity to the right hand of the receptacle portion 34, and reaches a beam splitter 36. The beam splitter 36 may be a dichroic mirror which reflects the infrared radiation having a wavelength longer than about 4 .mu.m but allows the infrared radiation having a wavelength shorter than about 4 .mu.m to transmit therethrough. The beam splitter 36 is slanted approximately 45.degree. with respect to the optical axis of the infrared radiation source 35. The infrared radiation is impinging on the beam splitter 36. Infrared radiation having a wavelength longer than 4 .mu.m is reflected and directed to the lead selenide (PbSe) detector 38 through a bandpass filter 37 which transmits wavelength in the range of about 4.3 .mu.m. Infrared radiation having a wavelength shorter than 4 .mu.m is, instead, transmitted through the beam splitter 36 and impinging on the lead selenide detector 40 through a bandpass filter 39 which transmits wavelength in the range of about 3.7 .mu.m.
Infrared spectrum of carbon dioxide gas is shown in FIG. 5. As seen from the spectrum diagram, the least transmittance of the carbon dioxide gas appears at its wavelengths near to 4.3 .mu.m, and the transmittance is approximately 100% at 3.7 .mu.m. In other words, most of infrared radiation having a wavelength of 4.3 .mu.m is absorbed by the carbon dioxide gas, while infrared radiation having a wavelength of 3.7 .mu.m is not absorbed. From this fact, it is seen that a concentration of the CO.sub.2 gas can be obtained by calculating a ratio of electrical signals, which are derived from the two detectors 38 and 40, propotional to the intensity of the infrared radiation impinging on them.
A heater h and a thermistor s are attached to a portion (of the receptacle portion 34) of the housing 30 where the housing comes in contact with the airway adaptor 31. The thermistor s senses temperature of the heater h. The heater h heats the airway adaptor 31 in order to avoid the condensation of water vapor on the inner surfaces of the windows 32 and 33 by highly humidified respiratory gases.
In the conventional art, as seen from the foregoing description, where the inner surfaces of the windows 32 and 33 are soiled with secretion, e.g., sputum, whose absorption amounts of the infrared radiation at 4.3 .mu.m and 3.7 .mu.m are different from each other, the absorption amount difference causes a false calculation of the carbon dioxide concentration.
In the conventional art, a heat source, a lamp, or the like is used for the infrared radiation source. If such an infrared radiation source suffers from degradation, drift or the like, its temperature varies. As a result, not only the intensity of the emitted light varies at 4.3 .mu.m and 3.7 .mu.m, but also the ratio of the intensity of the infrared radiation impinging on the two detectors 38 and 40 varies as shown by Planck's law of radiation.
As described above, the prior airway adaptor is high in cost to manufacture because expensive sapphire is used for the windows of the airway adaptor.
To avoid the codensation of water on surfaces of the windows of the airway adaptor, the airway adaptor is heated by the heater. The use of the heater causes an increase of power consumption, requires a long warm-up time. In other words, a quick measurement of the CO.sub.2 gas concentration from cold start is impossible in the prior analyzer.
The infrared radiation of two wavelengths, 4.3 .mu.m and 3.7 .mu.m, are used for measuring the carbon dioxide gas concentration. Therefore, the CO.sub.2 gas concentration measurement may be inaccuate by the soils of the windows of the airway adaptor, the degradation and drift of the infrared radiation source, and is instable.